Liquid-to-gas phase transition

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

The temperature at which a phase transition occurs is dependent on pressure (Figure 7). At atmospheric pressure (1 atm) the solid-to-liquid phase transition occurs at 0 °C and the liquid-to-gas phase transition occurs at 100 °C. If we increase the pressure, say to 100 atm, the solid-to-liquid phase transition occurs at a temperature slightly less than 0°C (—0.74°C) however, the liquid-to-gas phase transition occurs at a much greater temperature (312°C). If we decrease the pressure, say to 0.1 atm, the solid-to-liquid phase transition occurs at a temperature slightly greater than 0°C (0.004 °C) and the liquid-to-gas phase transition occurs at a lower temperature (46 °C). If we decrease the pressure further to below the triple point, there is no solid-to-liquid phase transition rather, the solid-to-gas phase transition occurs directly. At a pressure of 0.001 atm, the sublimation temperature is — 20.16°C. 

The same argument applies for the liquid-to-gas phase transition. In this case, the slope of the curve is usually higher because the difference in entropy between liquid and gas phases is much larger in magnitude than the difference in S between solid and liquid phases. However, the reasoning is the same, and equation 6.23 explains why liquids change to gas when the temperature is increased. 

Since an analyte and interferent are usually in the same phase, a separation often can be effected by inducing a change in one of their physical or chemical states. Changes in physical state that have been exploited for the purpose of a separation include liquid-to-gas and solid-to-gas phase transitions. Changes in chemical state involve one or more chemical reactions. 

The triple point is the location at which all three phases boundaries intersect. At the triple point (and only at the triple point), all three phases (solid, liquid, and gas) coexist in dynamic equilibrium. Below the triple point, the solid and gas phases are next-door neighbors, and the solid-to-gas phase transition occurs directly. 

Transport Properties. Viscosity, thermal conductivity, the speed of sound, and various combinations of these with other properties are called steam transport properties, which arc important in engineering calculations. The speed of sound is important to choking phenomena, where the flow of steam is no longer simply related to the difference in pressure. Thermal conductivity is important to the design of heat-transfer apparatus. See Heat-excliange Technology. Shaip declines ill each of these properties occur at the transition from liquid to gas phase, i.e., from water to steam. 

Noncowlent Interactions. An additional characteristic of electrospray ionization is its ability to monitor noncovalent interactions [31]. The ability of electrospray ionization to maintain the noncovalent structure of a biomolecule in its transition from the liquid to gas phase has provided a number of opportunities to study molecular dynamics previously not amenable to mass spectrometry. 

There are numerous ionization methods that allow formation of ions to carry out mass spectrometry however, in this chapter we will only focus on those most common in LC-MS. The challenge in coupling HPLC to mass spectrometry is that the chromatography operates with liquids and under high pressure, while the detector operates under high vacuum. The device between the chromatograph and the mass spectrometer is called the interface. Here, ionization and transition from liquid to gas phase of the compounds occur. The development of the first commercial available interfaces started as early as in the 1970s. Since then numerous interfaces have been introduced. Table 3.6 shows a list of current interfaces and their acronyms. 

If the system is not isolated but simply closed, then heat can enter or leave the system. In that case, the relative amounts of each phase will change. For example, in a system containing solid dimethyl sulfoxide (DMSO) and liquid DMSO at 18.4°C and atmospheric pressure, when heat is added to the system, some of the solid phase will melt to become part of the liquid phase. The system is still at chemical equilibrium, even though the relative amounts of phases are changing (which is a physical change). This is true of other phase transitions as well. At atmospheric pressure and 189°C, liquid DMSO can exist in equilibrium with gaseous DMSO. Add or remove heat, and DMSO will go from liquid to gas phase or from gas to liquid phase, respectively, while maintaining a chemical equilibrium. 

Solution The Clapeyron equation. Equation 4.28, provides the relation between temperature and pressure for a phase transition. We need to consider how it ap>plies in the case of a solid-to-liquid (or liquid-to-solid) phase transition. The approximation of Equation 4.32 to the molar volume for a phase transition is based in part on the fact that for a solid or a liquid, the molar volume is largely independent of temperature, as well as being a value much less than the molar volume of a gas. This means that for fusion or melting, the change in the molar transition volume in Equation 4.28 is usually well approximated as a constant (independent of T). The following steps are carried out with this approximation, as well as with the assumption that the transition enthalpy is independent of temperature. 

With a further increase in the temperature the gas composition moves to the right until it reaches v = 1/2 at the phase boundary, at which point all the liquid is gone. (This is called the dew point because, when the gas is cooled, this is the first point at which drops of liquid appear.) An unportant feature of this behaviour is that the transition from liquid to gas occurs gradually over a nonzero range of temperature, unlike the situation shown for a one-component system in figure A2.5.1. Thus the two-phase region is bounded by a dew-point curve and a bubble-point curve. 

Fig. 3. Phase diagram of van der Waals fluids. At temperature T > Tc, the pressure P is a convex function of the volume V, where Tc is the critical temperature. At T < Tc, a phase transition occurs from liquid to gas or gas to liquid. At T = T ( respectively...

We re all used to seeing solid/liquid and liquid/gas phase transitions, but behavior at the critical point lies so far outside our normal experiences that it s hard to imagine. A gas at the critical point is under such high pressure and its molecules are so close together, that it becomes indistinguishable from a liquid. A... 

Helium-4 Normal-Superfluid Transition Liquid helium has some unique and interesting properties, including a transition into a phase described as a superfluid. Unlike most materials where the isotopic nature of the atoms has little influence on the phase behavior, 4He and 3He have a very different phase behavior at low temperatures, and so we will consider them separately Figure 13.11 shows the phase diagram for 4He at low temperatures. The normal liquid phase of 4He is called liquid I. Line ab is the vapor pressure line along which (gas + liquid I) equilibrium is maintained, and the (liquid + gas) phase transition is first order. Point a is the critical point of 4He at T= 5.20 K and p — 0.229 MPa. At this point, the (liquid + gas) transition has become continuous. Line be represents the transition between normal liquid (liquid I) and a superfluid phase referred to as liquid II. Along this line the transition... 

D. Point A is located in the solid phase, point C is located in the liquid phase. Points B and D are located in the gas phase. The transition from solid to gas is sublimation and the transition from liquid to gas is vaporization. 

The concept of a reversible path seems to be fiction, but it is very real for phase transitions (e.g., solid-to-liquid, liquid-to-gas) involving a large number of particles—for example, Avogadro s3 number (NA) of particles this large system of particles achieves reversibility in the large number of interactions [(NA(NA—l)/2], which keeps the two phases in mutual coexistence. Now, we can write dU as a perfect differential in terms of natural variables (and state functions) S and V ... 

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